Revert of Remove the is_loading_ field from WebContentsImpl

content/browser/site_instance_impl.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_BROWSER_SITE_INSTANCE_IMPL_H_
 #define CONTENT_BROWSER_SITE_INSTANCE_IMPL_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include "base/macros.h"
 #include "base/observer_list.h"
 #include "content/browser/renderer_host/render_process_host_impl.h"
 #include "content/common/content_export.h"
 #include "content/public/browser/render_process_host_observer.h"
 #include "content/public/browser/site_instance.h"
 #include "url/gurl.h"
 
 namespace content {
 class BrowsingInstance;
 class RenderProcessHostFactory;
 
 class CONTENT_EXPORT SiteInstanceImpl : public SiteInstance,
                                         public RenderProcessHostObserver {
  public:
-  class CONTENT_EXPORT Observer {
+  class Observer {
    public:
     // Called when this SiteInstance transitions to having no active frames,
     // as measured by active_frame_count().
-    virtual void ActiveFrameCountIsZero(SiteInstanceImpl* site_instance) {}
+    virtual void ActiveFrameCountIsZero(SiteInstanceImpl* site_instance) = 0;
 
     // Called when the renderer process of this SiteInstance has exited.
     virtual void RenderProcessGone(SiteInstanceImpl* site_instance) = 0;
   };
 
   // SiteInstance interface overrides.
   int32_t GetId() override;
   bool HasProcess() const override;
   RenderProcessHost* GetProcess() override;
   BrowserContext* GetBrowserContext() const override;
@@ skipping 123 matching lines @@
   bool has_site_;
 
   base::ObserverList<Observer, true> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(SiteInstanceImpl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_SITE_INSTANCE_IMPL_H_